<template>
  <ul class="list">
    <li class="item"
     v-for="item of letters"
      :key="item"
      :ref="item"
    @touchstart.prevent="handleTouchStart"
    @touchmove="handleTouchMove"
    @touchend="handleTouchEnd"
    @click="handleLetterClick"
    >{{item}}</li>
  </ul>
</template>

<script>
export default{
  name:'CityAlphabet',
  props:{
    cities:Object
  },
  // 返回是letters的计算属性 A,B,C…
  computed: {
    letters() {
      const letters = []
      for (let i in this.cities) {
        letters.push(i)
      }
      return letters
    }
  },
  data(){
    return {
      touchStatus:false,
      startY:0,
      timer:null
    }
  },
  updated () {
    this.startY = this.$refs['A'][0].offsetTop
  },
  methods:{
    handleLetterClick(e){
      this.$emit('change',e.target.innerText)
    },
    handleTouchStart(){
      this.touchStatus = true
    },
    // 右侧字母拖动时实时滚动，点击完也实时变化
    handleTouchMove(e){
      if (this.touchStatus) {
        if(this.timer){
          clearTimeout(this.timer)
        }
        this.timer = setTimeout(() =>{
          const touchY = e.touches[0].clientY - 79
          const index = Math.floor((touchY - this.startY) / 20)
          if (index >= 0 && index < this.letters.length){
             this.$emit('change',this.letters[index])
          }
        },16)

      }
    },
    handleTouchEnd(){
      this.touchStatus = false
    }
  }
}
</script>

<style lang="stylus" scoped>
  @import '~styles/varibles.styl'
 .list
    display :flex
    flex-direction :column
    justify-content :center
    position :absolute
    top :1rem
    right :0
    bottom :0
    width :.4rem
    .item
      line-height :.4rem
      text-align :center
      color :$bgColor
</style>
